The present invention relates to an information coding system which is suitable for use in a digital transmission apparatus using differentially encoded coherent phase shift keying, and which can transmit a code signal (for example, a Hamming code) having a predetermined number of bits and a predetermined form, without deterioration of the error correcting capacity and the error detecting capacity peculiar to that particular code.
In a digital data transmission apparatus using differential phase shift keying, an input signal applied to a demodulator is first subjected to coherent detection, and then transmitted data is demodulated by a differential logic circuit (indicated by reference numeral 11H in FIG. 2). When the input data and output data of the differential logic circuit are identified as .PSI..sub.i and S.sub.i, respectively, the differential logic circuit performs the operation given by the following equation: EQU S.sub.i =.PSI..sub.i +.PSI..sub.i-1 ( 1)
where i indicates a time moment measured on a time unit necessay for shifting the input data by one bit.
Equation (1) indicates that the current input data .PSI..sub.i is added to the data .PSI..sub.i-1 of the previous unit time to obtain the output data S.sub.i. As is apparent from equation (1), when an error exists in the data .PSI..sub.i-1, both of the output data S.sub.i-1 and S.sub.i becomes erroneous. In other words, in a differential phase shift keying system, an error on one bit of input data results in errors in two consecutive bits of the demodulated data. Accordingly, in such a transmission apparatus as described above, for example, the Hamming code having a one-bit error correcting capacity and a two-bit error detecting capacity cannot exhibit its one-bit error correcting capacity effectively. Therefore, in conventional transmission apparatuses, a code which is more effective in error correcting capacity than the above-mentioned code, for example, a code having the two-bit error correcting capacity, has been employed. However, such a code requires a larger number of checking bits and thus becomes larger in both code length and redundancy. As a result, the transmission efficiency is lowered, and moreover the transmission apparatus becomes complicated.
Data transmission according to such a conventional differential phase shift keying system is disclosed in an article entitled "Error Statistics and Construction of Error Correcting Codes for Differentially Encoded CPSK Channels", IEEE Transactions on Information Theory, Vol. IT-24, No. 6, November 1978, pages 763 to 766.